net: ethernet: Add helper for set_pauseparam for Asym Pause

ethtool can be used to enable/disable pause. Add a helper to configure
the PHY when asym pause is supported.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>